The United States Public Land Survey (USPLS) began in 1785 with the
Land Ordinance. The USPLS effort was dedicated to surveying public lands that were unsettled at that time. An initial point
was first selected, and then a North-South line was drawn through
the initial point, and given a name. This North-South line is called a meridian. An East-West line is also drawn through the "initial point", and this line is called a base line.
The photo above shows me standing right next to Arizona's "initial
point". In the above photo, the baseline goes into the horizon, and the
meridian is going across the photo. Notice the road in the
distance. The center line of that road lines up with the baseline. The
road is very appropriately named "Baseline Road". The photo below is a
closeup of the "initial point" (it is located in the middle of the
cross, above). All land ownership, as well as all political boundaries
in Arizona, are based on this point. The map at the top left shows
the "Gila and Salt River Meridian", and
associated base line. From these two lines, the townships and ranges
are defined. Townships are stripes, six miles wide, running in an East-West direction, parallel to the baseline. Ranges are
also stripes, six miles wide, running in a North-South direction,
parellel to the meridian. The map of Apache County (note: Apache County
is located in the North-East corner of Arizona), on the bottom left,
has
the townships and the ranges shown. Note that the townships and ranges,
taken together, define a grid of six mile by six mile squares, based on
the meridian and the base line.

The townships are numbered according to their distance from the base
line, and according to whether they lie North or South of the base
line. So, for example, township 13N is the thirteenth township North of
the base line (it lies 13 * 6miles = 78miles North of the
base line).

Ranges are numbered according to their distance from the meridian, and
according to whether they lie East or West of the meridian. For
example, range 29E is the 29th range East of the meridian (it lies 29 *
6miles = 174miles East of the meridian).

As mentioned above, the townships and ranges divide the land into six
mile by six mile squares. Each of these 36 square mile subdivisions is
further divided into 36 sections, with each section being one square
mile. Typically, although not always, these sections are numbered as
follows:

N

W

6

5

4

3

2

1

E

7

8

9

10

11

12

18

17

16

15

14

13

19

20

21

22

23

24

30

29

28

27

26

25

31

32

33

34

35

36

S

When using this system to specify a section of land, always go from large to small. or example:

The state of Arizona, Gila and Salt River Meridian, Township 13N, Range 29E, Section 11

Specifies a square mile of land about 8 miles (as the crow flies) east of St. John's, Arizona.